The present invention relates to a working fluid used as a refrigerant in refrigerators and for other purposes. More particularly, the present invention relates to a refrigerant for providing an ultra-low temperature which does not possess any ozone destruction capability, thereby enabling to notably inhibit the influence thereof on the atmosphere of the earth (facilitating the reduction of the xe2x80x98green-housexe2x80x99 effect), and which can be easily used at the same capacity as that of a compressor used in conventional refrigeration rooms or chambers.
Recently, refrigeration rooms at a ultra-low cooling temperature of less than xe2x88x9250xc2x0 C. to xe2x88x9260xc2x0 C., which is lower than conventional refrigeration rooms, have been used with development of the biotechnology and food transportation systems, and demand of such refrigeration rooms are increasing.
In the field of biotechnology, cells, biological tissues and other biological substances have to be stably stored for an extended time of period at the above-mentioned ultra-low temperature to ensure their good survival activity rate after thawing. To satisfy this requirement, the refrigeration rooms used for cells and other biological substances need to have highly increased refrigerating power, along with a high reliability and a low maintenance cost. Further, in order to enable biotechnology to be applied in hospitals and other institutions in addition to application in laboratories, the refrigeration rooms have to be constructed simply, at low cost; and also need to be easy to operate.
Similar problems also occur in food transportation systems etc. To maintain freshness of the food for a long period, the refrigeration rooms used in the transportation system must have high refrigerating power without any problems such as system failure, along with easy maintenance and low operation cost.
Under these circumstances, it is preferable to provide refrigeration rooms in which a refrigerant is repeatedly used in refrigeration cycles. However, refrigerants capable of providing an ultra-low cooling temperature of less than xe2x88x9250xc2x0 C. can not be easily liquefied at room temperature, because the critical pressure thereof is generally increased with the reduction of the standard boiling point, and have a low critical temperature.
Hitherto, a refrigerator unit, based on a multistage cooling cycle, using two or more refrigerants having different boiling points has been used as a refrigerator for ultra-low temperature. Namely, by using a refrigerant having a high boiling point capable of being liquefied at room temperature for a refrigeration process that liquefies a refrigerant having a lower boiling point, an ultra-low temperature can be obtained.
A multistage cooling cycle-based refrigerator unit is illustrated in, for example, FIG. 1 in which two types of refrigerants are used and two sets of refrigerator units are operated with two compressors at two stages, respectively.
In the illustrated refrigerator room, a first refrigerant is compressed in a high temperature side-positioned compressor 1 and the gaseous compressed refrigerant is subjected to heat radiation and cooling in a high temperature side-positioned condenser 3 provided with a fan 2, thereby producing a liquefied first refrigerant. The liquefied first refrigerant is guided through a capillary tube 5 to an outer tube 11 of the double tubed heat exchanger 10. After the vaporized first refrigerant in the outer tube 11 is used to cool a second refrigerant in an inner tube 12 of the heat exchanger 10, the first refrigerant is returned to the high temperature side-positioned compressor 1. In the above process, the reference numerals 6 and 7 represent a drier and a liquid separator (accumulator), respectively.
A second refrigerant, after being compressed in a low temperature side-positioned compressor 20, is led into an inner tube 12 of the heat exchanger 10, and is cooled and liquefied with the first refrigerant. The liquefied second refrigerant is guided through a capillary tube 15 to a low temperature side-positioned evaporator 30. In the evaporator 30, the second refrigerant is vaporized under a reduced pressure to thereby cool the interior of the refrigerator room. The used second refrigerant is returned again to the compressor 20. In the above process, the reference numerals 26 and 27 represent a drier and an oil separator for removing mist-like oil, respectively.
In the above illustrated refrigeration system, it becomes possible to provide an ultra-low temperature which has a system power and capacity comparable to conventional refrigeration rooms. However, because it is constructed from two sets of refrigerators, the total size of the refrigeration system is increased and has a complicated structure, thereby causing difficulty in maintenance, substantially increasing the cost of the refrigeration room.
Alternatively, as illustrated in FIG. 2, a single compressor multicycle refrigeration system in which a mixture of two or more refrigerants having different properties such as different boiling points is used in combination with a single compressor has been researched.
In the illustrated refrigeration system, three types of refrigerants are previously mixed to obtain a mixed refrigerant. The mixed refrigerant is compressed in a compressor 40 provided with a fan 2, followed by being subjected to heat radiation in a condenser 41 to thereby liquefy a first refrigerant having the highest critical temperature.
The liquefied first refrigerant is then separated in a liquid separator 45 to remove and recover therefrom an mist-like oil contaminated by the compressor 40 and return the oil to the compressor 40. The separated first refrigerant is vaporized in a heat exchanger 50 to simultaneously cool and liquefy a gaseous second refrigerant having a lower critical temperature than that of the first refrigerant. The second refrigerant liquefied in the heat exchanger 50 is separated in a liquid separator 46 and then vaporized in a heat exchanger 51 in which a third refrigerant having the lowest critical temperature is cooled and liquefied with the vaporized second refrigerant. The third refrigerant liquefied in the heat exchanger 51 is vaporized in an evaporator 55. The thus the produced vapor of the third refrigerant is used to cool the interior of the refrigeration room to a predetermined ultra-low temperature.
In the above refrigeration system, the first to third refrigerants vaporized in the heat exchangers 50 and 51 and the evaporator 55 are returned through a common return pipe 61 to the compressor 40.
Using the illustrated refrigeration system, it becomes possible to reduce the amount of machinery utilized in the refrigeration room because only one compressor is included therein. However, contrary to this advantage, the flow circuit for circulating the three refrigerants is complicated and thus the total size of the refrigeration room is unavoidably increased along increased difficulty of the maintenance.
In addition to the improvement of the refrigeration system, an improvement of the refrigerant used as the working liquid therein has been also made. Hitherto, fluorohydrocarbons which are generally referred to as xe2x80x9cflonsxe2x80x9d have been used as refrigerants. However, due to recent evidence proving that flon gas can cause destruction of the ozone layer adding to global warming, such flons are prohibited from being used as refrigerants. Namely, use of xe2x80x9cspecified flonsxe2x80x9d capable of causing notable ozone destruction and flons capable of adding substantially to general global warming can not be used under established regulations. Therefore, it is highly desirable to develop a novel refrigerant which has zero ozone destruction properties and a negligible effect on global warming.
At present, many types of the refrigerants which can be used without causing any adverse effect on the environment and which can show excellent properties comparable to those of the conventional flons have been proposed as an alternative to the above-described specified flons and other flons.
For example, a two component or three component refrigerant including perfluoroethane, ethane and trifluoromethane along with 1 to 10% by weight of propane and butane having a good affinity with lubricating oil has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-186765. This reference discloses that the return of the lubricating oil to the compressor can be accelerated with use of propane and butane, however, the cooling temperature and the pressure applied during the liquefication process is not taught.
Further, a mixed refrigerant including trifluoromethane and ethane or hexafluoromethane and ethane, and having a reduced standard boiling point of xe2x88x9290xc2x0 C. or less has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-48563. However, due to a low critical temperature and a high critical pressure, the mixed refrigerant can not be used in conventionally single cycle refrigeration rooms.
Furthermore, the applicant of this application has suggested a refrigerant having an ozone destruction capability of zero level which inhibits global warming in Japanese Unexamined Patent Publication (Kokai) Nos. 5-306391 and 7-48562. JPP""391 teaches the use of a refrigerant mixture of dihydrotetrafluoroehane (CH2-FCF3; generally referred to as xe2x80x9cHFC-134axe2x80x9d or xe2x80x9cR-134axe2x80x9d) and trifluoromethane (CHF3; generally referred to as xe2x80x9cHFC-23xe2x80x9d or xe2x80x9cR-23xe2x80x9d), and JPP""562 teaches use of a refrigerant mixture of dihydrotetrafluoroehane (R-134a) and perfluoroethane (C2F6; generally referred to as xe2x80x9cFC-116xe2x80x9d or xe2x80x9cR-116xe2x80x9d).
When additives such as propane, butane or other hydrocarbons are added to refrigerants described in JPP""391 and ""562, it becomes possible to reduce the internal temperature of the refrigeration room to less than xe2x88x9250xc2x0 C. at a discharge pressure of about 20 Kg/cm2, which can be applied effectively to conventional single cycle refrigeration rooms. Therefore, the above-mentioned refrigerants can be advantageously used in refrigerators and other devices; however, to satisfy the above-described requirements for refrigeration in the field of biotechnology, food transportation systems etc., it is more desirable to provide an improved refrigerant capable of ensuring a remarkably reduced temperature of the refrigeration room which is significantly lower than xe2x88x9250xc2x0 C.
In view of the above-described problems of the prior art refrigerants, one object of the present invention is to provide a working fluid which can ensure an ozone destruction capability of zero level and inhibited global warming properties which can achieve a satisfactory ultra-low temperature by means of conventional compressors. Particularly, the present invention is directed to provide a refrigerant which can easily realize a refrigeration ambient temperature of less than xe2x88x9260xc2x0 C., when the refrigerant is used in the conventional single cycle refrigeration rooms.
DISCLOSURE OF INVENTION
In order to achieve the above mentioned object, there is provided a refrigerant for providing an ultra-low temperature in which the refrigerant includes trifluoromethane (CHF3), perfluoroethane (C2F6), and a conventional fuel selected from the following fuels: propane, butane, or a mixture of propane and butane.
Preferably, the trifluoromethane and the perfluoroethane are contained in a mixing ratio of 70% to 15% by weight of trifluoromethane, and 30% to 85% by weight of perfluoroethane.
Preferably, the propane is included by an amount 55% to 95% by weight, the butane is included by an amount 50% to 90% by weight, or the mixture of propane and butane is included by an amount 35% to 70% by weight.
According to another aspect of the present invention, there is provided a refrigerant for providing an ultra-low temperature, in which the refrigerant comprises trifluoromethane (CHF3), propane and butane.
Preferably, the refrigerant comprises 60% to 15% by weight of trifluoromethane, 16% to 34% by weight of propane and 24% to 51% by weight of butane.
According to another aspect of the present invention, there is provided a refrigerant for providing an ultra-low temperature, in which the refrigerant comprises trifluoromethane (CHF3) and butane.
Preferably, the refrigerant comprises 50% to 15% by weight of trifluoromethane and 50% to 85% by weight of butane.
According to another aspect of the-present invention, there is provided a refrigerant for providing an ultra-low temperature, in which the refrigerant comprises perfluoroethane (C2F6), propane and butane.
Preferably, the refrigerant comprises 60% to 20% by weight of perfluoroethane, 16% to 32% by weight of propane, and 24% to 48% by weight of butane.
According to another aspect of the present invention, there is provided a refrigerant for providing an ultra-low temperature, in which the refrigerant comprises perfluoroethane (C2F6) and butane.
Preferably, the refrigerant comprises 55% to 20% by weight of perfluoroethane and 45% to 80% by weight of butane.